Differentially pumped air bearing systems are known; see for instance Fox U.S. Pat. No. 4,191,385 issued Mar. 4, 1980 and Lewis et al. U.S. Pat. No. 4,425,508 issued Jan. 10, 1984. Present FIG. 1, identical to FIG. 2 of Lewis et al., shows in a plan view the top of the air bearing structure which supports a moving planar surface (not shown, see Lewis et al.). The air bearing plate 17 defines around its periphery a plurality of H-shaped grooves 41. The groove pattern follows the outer perimeter of the bearing plate 17, in a square contour with rounded corners. At the middle of the cross-bar of each H-groove there is a metering valve 42 which controls and limits the flow of air into that groove. Air under pressure is provided to the H-shaped grooves through the metering valves 42 from a manifold (not shown) and this air flow provides an air cushion between the face of the plate 17 and the opposing face of the bearing surface (not shown). This structure is intended for use in a vacuum chamber, for instance in an electron beam lithography system. Since some of the air from the air bearing necessarily flows toward the evacuated area which is region 46, two pump-out slots 43 and 45 are located inboard of the groove 41. Pump-out slots 43 and 45 are provided with successively greater levels of vacuum pumping to scavenge the inwardly escaping air and prevent it from reaching vacuum region 46. The amount of escaping air is held to a minimum by observing close tolerances between the facing surfaces of the plate 17 and the opposing surface.
Hence this air bearing is separated from the vacuum region by the pump-out slots so that the air flowing into the bearing is pumped out through the pump-out slots before it can reach the central vacuum region enclosed by the pump-out slots. Typically a moving stage is supported by this bearing for XY movement, where the XY movement with regard to FIG. 1 is in the plane of the drawing. The pressure in the vacuum region 46 is maintained by conventional vacuum pumps. To maintain that pressure, the pumping capacity of the pumps must be adequate to handle the added burden of the air (or other gas) escaping from the air bearing. Hence it is recognized that it is advantageous to reduce the air flow to the vacuum, and hence in FIG. 1 there are two pump-out slots.
Moreover, the air from the air bearing flows through the gap between the plate 17 and the opposing bearing surface on which it rides. Since this gap is very small, the air flow to the vacuum region is advantageously reduced. However, the smaller the gap the tighter the necessary mechanical tolerance on the bearing plate 17 and the opposing surface, which substantially increases the manufacturing costs. A typical air bearing gap is 5 microns (.mu.m) which requires precision machining over a relatively large surface area; this substantially increases manufacturing costs, especially because two such large precision machined bearing surfaces are needed. Since the bearing stiffness is a function of this gap, adjusting the gap purely to control the air flow is often impractical. Hence an air bearing suitable for use in a vacuum (not just surrounding a vacuum) and which is also economical to fabricate is needed.